A P-Number is an alphanumeric designation assigned to a group of materials. These materials are grouped based on their similar weldability and mechanical characteristics.
P-Numbers are assigned to base metals to reduce the number of welding and brazing procedure qualifications required to perform work.
P-Numbers are made of numbers and sometimes include letters. It is important to note that each P-Number is considered a separate and unique number. For example, base metals assigned P-No. 5A are an entirely different group than P-No. 5B.
ASME: P-Number (P for Procedure Number)
ISO: Group Number
AWS B2.1: M-Number (M for Material Number)
AWS D1.1, D1.2, D1.6: Group Number
Code boundaries mainly fall along different industries, and materials unique to those industries will sometimes not overlap from one governing code body to the next. For example, some carbon steels like I-Beam listed in D1.1 will not be listed in ASME, even though they might both be considered to be plain carbon steel.
P-Numbers have been laid out in such a way that there are some identifiable alloy patterns.
The following is the comprehensive list of P-Numbers, their general material description, and some typical material specification examples.
When a procedure is qualified, a PQR or Procedure Qualification Record is created to document all of the essential variables. The WPS is then written to include Code allowed ranges based on what materials were used on the PQR. A summary of the ranges permitted on the WPS based on the PQR P-Number is contained in the following table. The WPS author can restrict these ranges or take full advantage of what the Code allows; see ASME Section IX para. QW-424.1 for details.
When welders take a qualification test, the base metal used during the test is recorded as an “Actual Value.” Based on the actual P-Number, the Code specifies a “Range Qualified” that the welder is then certified to weld on. P-Number is one of many essential variables listed on the WPQ in the Range Qualified column; all must be observed to conform to Code requirements.
Per ASME Section IX para. QW-423.1 when a base metal shown in the left column is used for welder qualification, the welder is qualified to weld all combinations of base metals shown in the right column, including unassigned metals of similar chemical composition to these metals. This range of values needs to be clearly stated on the WPQ.
In 2009, S-Numbers were removed from Section IX. S-Numbers were assigned to materials that were acceptable for use by the ASME B31 Code or had been deemed acceptable Code Cases within the Boiler and Pressure Vessel Code, but which were not included in ASME Section II. They are no longer used, as base metals previously assigned S-Numbers were assigned P-Numbers and Group Numbers, and a single system is now used
Ferrous base metals have been assigned Group Numbers, which create subsets of P-Numbers. Group numbers are primarily required when WPS’s are required to be qualified by impact testing.
Both the materials specification and the Type or Grade are essential information in having the correct material. The material specification essentially calls out the product form. The Type or Grade of that specification is more aligned to the metallurgy of that product form and heavily influences the P-Number as well as the Group Number of the material.
There are some material specifications that are almost exclusively one P-Number, such as SA-240 & SA-312, where almost all of the materials in those specifications are P-No. 8. This can sometimes lead to the incorrect assumption that the specification alone is a sufficient amount of information to determine a material’s P-Number.
Here are some examples of materials where Type or Grade can change the P-Number of the material:
The following is an example of how Type or Grade can change the Group Number of the material:
In general, ferrous metals contain iron; non-ferrous metals, on the other hand, do not contain iron. ASTM and ASME have created a prefix convention for identifying ferrous and non-ferrous materials.
A/SA is used in the specification to define those materials which are ferrous.
B/SB is used in the specification to define those materials which are non-ferrous.